This invention pertains to a composition of matter that is photosensitive, and in particular, to a material that is able to preserve a spatial modulation in three dimensions when exposed to a spatially varying illumination. The unique composition is designed to allow the use of a mixture of photopolymer, or other photoreactive material, and a microscopically porous rigid transparent structure, such as an aerogel, to form a holographic media of variable thickness which eliminates shrinkage or other shape distortions found in photopolymer based devices. A method of producing a holographic data storage apparatus utilizing this material is also disclosed.
A large number of materials and devices are currently used to record holographic images or interference patterns. In a holographic recording process, an object beam is mixed with a reference beam from the same laser. The summation of the two fields gives rise to a unique spatially varying amplitude pattern. This pattern, when recorded by an amplitude sensitive medium, is the hologram. The hologram may preserve both amplitude and phase information presented in the mixture of the two beams. A three dimensional image may be preserved and reconstructed by the illumination of the hologram with the conjugate of the reference beam. In addition to the storage of imagery, data may be placed in the object beam by means of a spatial pattern of dark and light areas. This data is then recorded as a page in the holographic media and may be later read by the application of the reference beam or its conjugate to the same location and at the same angle as the original object beam. If shrinkage occurs during the original or subsequent recordings, the readout of the desired page may be erroneous, nonexistent or directed to the incorrect location for readout. Current media show limitations in the allowable thickness of a holographic exposure to small fractions of millimeter, whereas the more significant advantages of three dimensional data storage are to be found in bulk materials with thicknesses in excess of several millimeters. In other applications, a holographic media's natural shrinkage must be compensated for at the time of exposure, such as in the production of holographic notch filters made via interferometric exposure.
Photopolymers have been used extensively as holographic recording materials. They are generally composed of a mixture of polymer molecules, monomers and photoactive substance that catalyze polymerization of the monomers in response to illumination. They have been used as stand alone materials of relatively thin designs, primarily as films or coatings on a rigid or flexible substrate. Thick designs have not been successfully used because of internal structural changes and subsequent image loss or data distortion caused by microscopic shape distortions and bulk shrinkage of the polymer component. What has not been done in this area is a photoactive material that can be used in varying thickness without distortion as a holographic recording media for numerous applications in optics and photonics.
Another, completely unrelated, class of materials are low density microscopically porous glasses where the internal structures are rigid but composed of nanometer scale structures. This class includes porous glasses, aerogels and nanostructured materials. Porous glasses, made by selective chemical leaching of silica glasses, have been used as structural supporting elements in other experimental composite holographic media, but have relatively low porosity, generally less than 50% and usually less than 20%, whereas aerogels are the least dense man-made substances, with porosities greater than 80%. Aerogels, sometimes referred to as "solid smoke" have been used as thermal insulating materials; as porous electrodes in electrical storage devices; in devices for collecting cosmic dust; as electromagnetic radiation absorbers for stealth type applications; as a catalyst support surfaces; in ion exchange matrices; for fuel cell diffusion barriers and in gas and liquid filtration. One reference that details the history and applications of aerogels is "Aerogels" by Dr. Jochen Fricke, published by Springer-Verlag, Berlin, in 1986 ISBN 3-540-16256-9. Methods of fabricating aerogels have been described in the U.S. Pat. No. 3,672,833, issued to S. J. Teichner, for a "Method of Preparing Inorganic Aerogels", which issued on Jun. 27, 1972 and the Patent issued to Yokogawa et al, U.S. Pat. No. 5,496,527 for a "Process for Forming A Hydrophobic Aerogel", which issued on Mar. 5, 1996.
It is the combination of these two previously unrelated classes of materials, photopolymers and aerogels, that is the subject of this invention. The photosensitivity of a photopolymer type material is coupled with the inherent rigidity and porosity of an aerogel type material to form a new composition of matter that is a nanostructured holographic media with unique properties and utility.
Applications of the composite holographic media and numerous and broad in their scope. Beside the uses indicated in the previous descriptions herein for hologram image recording and holographic data recording, it is clear that all classes of holographic optical elements (HOE's) and diffractive optical elements (DOE's) will be possible with the present invention. Thick media that is stable is an advantage in these fields as it is in the production of holographic gratings, diffraction gratings and wavelength selective filters of all types. Narrow bandwidth filters such as notch filters and filters usable for dense wavelength division multiplexing (DWDM) fiber optic or free space optical communications systems are of significant commercial value.
The composition of matter that is the subject of the present invention may be composed from numerous types of photosensitive compounds or mixtures in combination with any of a variety of porous structures and still fall within the scope of the invention. Mechanisms of recording in the resulting composite can be the result of any type of physical phenomena, regardless of a capacity to shrink but benefiting from having a potentially thick and rigid, or semi-rigid, internal structure. In the case of most photopolymers the physical mechanism that produces the spatial modulation that is the hologram is the light initiated polyerization of unbound monomers. It is the illumination generated spatially varying polymerization and subsequent diffusion of unpolymerized monomers that causes both the desired variation in index of refraction and the unwanted shrinkage. Other mechanisms of causing illumination generated spatial modulation may result in variations in birefringence or polarization rotation, optical absorption or wavelength specific optical absorption, and reflectivity.
Clearly, it is desirable for a material of this type to be very adaptable. At the same time, the material should be easy to manufacture and be produced of cost effective precursors. It is the object of this invention to set forth a composite holographic media and a method of producing the same which avoid the disadvantages and previously mentioned limitations of typical current media.